Fitting polytomous Rasch models in SAS

The item parameters of a polytomous Rasch model can be estimated using marginal and conditional approaches. This paper describes how this can be done in SAS (V8.2) for three item parameter estimation procedures: marginal maximum likelihood estimation, conditional maximum likelihood estimation, and pairwise conditional estimation. The use of the procedures for extensions of the Rasch model is also discussed. The accuracy of the methods are evaluated using a simulation study.     

Empfehlungen des Arbeitskreises 6.1 “Geotechnik der Deponiebauwerke” der Deutschen Gesellschaft für Geotechnik e. V. (DGGT)

Der Arbeitskreis “Geotechnik der Deponiebauwerke” hat in 5 Themengruppen bisher 76 Empfehlungen zur Planung, zum Bau und zur Qualitätsüberwachung von Abfalldeponien veröffentlicht. Der diesjährige Bericht zu den Arbeitsergebnissen stellt den Bezug zwischen der im Juli in Kraft getretenen Verordnung zur Vereinfachung des Deponierechts und den GDA‐Empfehlungen her. Recommendations of the DGGT Committee “Geotechnics of Landfill Structures”. The Technical Committee “Geotechnics of Landfill Structures” of the German Geotechnical Society published up to now 76 recommendations (GDA‐Geotechnical Landfill Recommendations) structured in 5 main issues. This report on the recent results of the working group delivers the link between the requirements of the new German Directive on the Landfilling of Waste and the detailed technical standard described in the GDA‐Recommendations.     

Einheit von Wissenschaft und Kunst im Brückenbau: Hellmut Homberg (1909–1990) – Leben und Wirken (Teil I)

Am 5. September 2009 wäre Hellmut Homberg einhundert Jahre alt geworden. Dies ist Anlass, an sein vor allem den Brückenbau bereicherndes Wirken zu erinnern und ein, wenn auch nicht vollständiges, Werkverzeichnis zu erstellen. Das gilt sowohl für die unter seiner maßgebenden Mitwirkung entworfenen Brücken als auch für seine völlig neuartigen, die statischen Berechnungen in der Praxis erleichternden, streng theoretisch hergeleiteten und eine zutreffende Bemessung der Kreuzwerke und orthotropen Fahrbahnplatten ermöglichenden Tafel‐ und Tabellenwerke. Teil I schildert Homberg s beruflichen Weg und versucht, ein Porträt zu zeichnen; Teil II ist seinen theoretischen Untersuchungen gewidmet, und Teil III geht auf besondere Brücken ein, und. Harmony between science and art in bridge‐building: Hellmut Homberg (1909–90) – life and work (part I). Hellmut Homberg would have been 100 years old on 5 September 2009. This is an opportunity to look back on his work which so enriched the world of bridge‐building in particular, and also a chance to compile a catalogue of his work, albeit incomplete. This applies to the bridges in which he played an influential role in their design and also to his books of mathematical and design tables that enabled the accurate design of beam grids and orthotropic bridge decks. The tables with their rigorous theoretical background were at the time quite new and eased structural calculations in everyday practice. Part I describes Homberg's professional career and attempts to draw a portrait of the man; part II is devoted to his theoretical studies, and part III deals with particular bridges.     

Cooperative Wireless Networking Beyond Store-and-Forward

In future wireless networks devices may cooperate to form logical links. Each of these links may consist of several independent physical channels which are shared by the cooperating partners. Even without multiple antennas this cooperation provides diversity in time and space. This so-called cooperation diversity increases the robustness of the link vs. fading and interference. After surveying approaches in cooperation diversity we focus on optimizing its performance by combining several cooperation schemes and by integrating cooperation into space-time coding. For multiple scenarios, we further discuss the factors and benefits introduced by user cooperation and how cooperation-aware resource allocation can be employed to further increase the performance of cooperative networks. When it comes to implementation, the question arises how cooperation can be integrated efficiently into existing wireless networks. A case study for 802.11-based WLANs reveals the issues that need to be solved in order to deploy cooperative techniques. We provide an overview of the state of the art in implementing cooperation approaches, analyze how appropriate these approaches solve the issues, and, where appropriate, point out their deficiencies. We conclude with a road map for future research necessary to tackle these deficiencies for the practical implementation of cooperation in next generation mesh, WLAN, WMAN, and cellular standards.

Characterisation of a setup for mixed beam exposures of cells to 241Am alpha particles and X-rays

Exposure of humans to mixed fields of high- and low-linear energy transfer (LET) radiation occurs in many situations-for example, in urban areas with high levels of indoor radon as well as background gamma radiation, during airplane flights or certain forms of radiation therapy. From the perspective of health risk associated with exposure to mixed fields, it is important to understand the interactions between different radiation types. In most cellular investigations on mixed beams, two types of irradiations have been applied sequentially. Simultaneous irradiation is the desirable scenario but requires a dedicated irradiation facility. The authors have constructed a facility where cells can be simultaneously exposed to (241)Am alpha particles and 190-kV X-rays at 37°C. This study presents the technical details and the dosimetry of the setup, as well as validates the performance of the setup for clonogenic survival in AA8 Chinese hamster ovary cells. No significant synergistic effect was observed. The relative biological effectiveness of the alpha particles was 2.56 for 37 % and 1.90 for 10 % clonogenic survival.     

Effect of film thickness, type of buffer layer, and substrate temperature on the morphology of dicyanovinyl-substituted sexithiophene films

The influence of film thickness, type of buffer underlayer, and deposition substrate temperature on the crystal structure, microstructure, and morphology of the films of dicyanovinyl-substituted sexithiophene with four butyl-chains (DCV6T-Bu₄) is investigated by means of X-ray diffraction (XRD) and X-ray reflectivity methods. A neat Si wafer or a Si wafer covered by a 15 nm buffer underlayer of fullerene C₆₀ or 9,9-Bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF) is used as a substrate. The crystalline nature and ordered molecular arrangement of the films are recorded down to 6 nm film thickness. By using substrates heated up to 90 °C during the film deposition, the size of the DCV6T-Bu₄ crystallites in direction perpendicular to the film surface increases up to value of the film thickness. With increasing deposition substrate temperature or film thickness, the DCV6T-Bu₄ film relaxes, resulting in reducing the interplane distances closer to the bulk values. For the films of the same thickness deposited at the same substrate temperature, the DCV6T-Bu₄ film relaxes for growth on Si to BPAPF to C₆₀. Thicker films grown at heated substrates are characterized by smaller density, higher roughness and crystallinity and better molecular ordering. A thin (up to about 6 nm-thick) intermediate layer with linear density-gradient is formed at the C₆₀/DCV6T-Bu₄ interface for the films with buffer C₆₀ layer. The XRD pattern of the DCV6T-Bu₄ powder is indexed using triclinic unit cell parameters.     

Time and temperature dependent piezoresistance of carbon nanofiller/polymer composites under dynamic load

In this study, the behaviour of carbon nanotube/epoxy and carbon black/epoxy composites under dynamic load is studied via dynamic mechanical thermal analysis (DMTA) in combination with DC electrical resistivity measurements. DMTA measurements are carried out at fixed temperature whilst the dynamic loading frequency is varied. With this procedure, a loading frequency-dependence of the phase shift between DC electrical resistance and mechanical elongation (δ _R–ε) is observed, although the force and elongation of the sample are still in phase. Moreover, the magnitude of this phase shift, as well as the amplitude of the DC electrical resistance change shows a clear dependence on the initial electrical conductivity of the samples. In addition, temperature sweeps are carried out to investigate the temperature dependency of the piezoresistance of the samples. An abrupt change in their sensitivity is observed as soon as the glass transition of the polymer is reached. However, the trend of the resistance change beyond the glass transition is substantially different between the nanocomposites containing carbon black and carbon nanotubes, revealing a strong influence of the network characteristics on the piezoresistive behaviour of these novel materials.     

Analysis of martensitic transformation in 304 type stainless steels tensile tested in high pressure hydrogen atmosphere by means of XRD and magnetic induction

Tensile specimen of several 304 type stainless steels tested under pressurized hydrogen and helium atmospheres were investigated with the focus on the γ → α′ transformation as a function of Ni content. Martensite contents on the fracture surfaces increased with decreasing Ni content and were independent of the test atmosphere (He or H₂) despite different macroscopic plastic deformations. This was attributed to similar plastic deformations at the crack tip which governs the γ → α′ transformation at the fracture surface. The severity of hydrogen environment embrittlement was quantified by RA measurements which is a measure of the maximum macroscopic plastic deformation. RA values in H₂ decrease with decreasing Ni content and RA is almost exactly inverse proportional to the martensite content measured by Feritscope in the uniform elongation area. This implies that the influence of hydrogen of the steels investigated here is dominated by surface effects.     

Role of eye movements in chromatic induction

There exist large interindividual differences in the amount of chromatic induction [Vis. Res. 49, 2261 (2009)]. One possible reason for these differences between subjects could be differences in subjects' eye movements. In experiment 1, subjects either had to look exclusively at the background or at the adjustable disk while they set the disk to a neutral gray as their eye position was being recorded. We found a significant difference in the amount of induction between the two viewing conditions. In a second experiment, subjects were freely looking at the display. We found no correlation between subjects' eye movements and the amount of induction. We conclude that eye movements only play a role under artificial (forced looking) viewing conditions and that eye movements do not seem to play a large role for chromatic induction under natural viewing conditions.